Power vs. Fuel

[Prior Engagement]

O.K. here is the deal. After surveying several very versed motorheads, I have arrived at a point where I am looking for some additional educated feedback. Not looking for opinions here guys. Know where I can get plenty of them already! :P :wink:

If you have three engines and they all produce 330 HP, which one is going to burn the most fuel (GPH) to achieve the 330 HP. Note we want to produce the same HP. The three engines are a 6 cyl. in-line diesel, small block V8 gas, and a big block V8 gas? This struck me while reading the Boating Mag. report on the new 30' Luhrs. In this article, they also mention the 30' Rampage. According to the articel the Rampage and Luhrs were both equipped with 315 HP Yanmars. The Rampage got 1 mpg (not sure at what speed or rpms) and the Luhrs ' best was 1.1 nmpg at 22.8 knots and 3000 rpms.

What do you guys think? I know that the diesel will produce way more torque and be more economical to run, but jsut wondering o nthe power vs. fuel thing. Way I figure it, you got to feed them ponies.

[No Tacks]

If you like to make long runs offshore alot then diesel would be your choice.. :D

[CAPT GUTS]

Here's the answer: the big block is going to burn the most. Then the small block. Then the diesel. The small block doesn't compare to the other two. You may have the hp, but no torque. So foget the small block.
The diesel is so much bigger(in power) than the big block that you could get away with a 250 hp diesel rather than the big block. A 330hp diesel is a big baby!!
The diesel will burn 40 percent less than the big block. Depends how you run it. If you are going to get the full power of the big block, oh, brother. The fuel consumption will be outta site. You'll be turning 4000 rpm. This translates to 30 gal an hour. Forget it!!
Go to the Bertram websit, they go thru this all the time because a lot of the boats have switched to diesel and they're fanatics.

[Billt]

To do the comparison you want to do you need to compare torque, and even then you do not have a true picture of the differences between the engines. The best way to compare engines in to look at those graphs that plot power and fuel use at a given RPM. When you are trying to get a heavy hull on plane when the engines make the power is very important. A big block Gas may give you 330 HP at 5800 RPM but what does it put out at 1800 RPM?

I did see a fuel useage rule of thumb that seams to hold true for most motors. Max fuel burn for gas is about 10% of HP and about 5% for Diesel. Not that Max burn does much for you.

[Engineman]

Billt is correct.
Horsepower is the number that engine builders try to throw at us for lack of education. Torque is the key. Compare the charts of torque and you will find the answer. After that the hull construction is key. Some will plane faster, some will cruise with less effort. For example: If you put a 330hp small block in a HumV and run the quarter it will be slower than the exact engine in a Vega. The heavier boat will use more of the torque to stay going while the lighter will use it to go faster. One of those engineering things that say there is a given amount of power to overcome friction. Seems to hold true.
Best comparison is for the exact hull design (with the comfort or performance you want) against the various engine options.

[Prior Engagement]

Thanks for the feedback so far. Been a long cold winter and I have been spending a lot of time thinking about what to do power wise with my future upgrade.

The 1 mpg thing definitely has me concerned as the fun trips with one or two guys becomes over twice as expensive as going on the PE with a single small block getting 2 to 2.25 mpg.

We had some discussion over some beers this weekend about the subject. Stroke, torque, displacement, etc. were thrown into the equations.